netdev - Re: [PATCH net-next v2 01/17] net: sched: refactor mini_qdisc_pair

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Message-ID: <vbfftuxzb3r.fsf@mellanox.com>
Date:   Sun, 16 Dec 2018 16:32:13 +0000
From:   Vlad Buslov <vladbu@...lanox.com>
To:     Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>
CC:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH net-next v2 01/17] net: sched: refactor
 mini_qdisc_pair_swap() to use workqueue

On Thu 13 Dec 2018 at 23:32, Cong Wang <xiyou.wangcong@...il.com> wrote:
> On Tue, Dec 11, 2018 at 2:19 AM Vlad Buslov <vladbu@...lanox.com> wrote:
>>
>> As a part of the effort to remove dependency on rtnl lock, cls API is being
>> converted to use fine-grained locking mechanisms instead of global rtnl
>> lock. However, chain_head_change callback for ingress Qdisc is a sleeping
>> function and cannot be executed while holding a spinlock.
>
>
> Why does it have to be a spinlock not a mutex?
>
> I've read your cover letter and this changelog, I don't find any
> answer.

My initial implementation used mutex. However, it was changed to
spinlock by Jiri's request during internal review.

>
>>
>> Extend cls API with new workqueue intended to be used for tcf_proto
>> lifetime management. Modify tcf_proto_destroy() to deallocate proto
>> asynchronously on workqueue in order to ensure that all chain_head_change
>> callbacks involving the proto complete before it is freed. Convert
>> mini_qdisc_pair_swap(), that is used as a chain_head_change callback for
>> ingress and clsact Qdiscs, to use a workqueue. Move Qdisc deallocation to
>> tc_proto_wq ordered workqueue that is used to destroy tcf proto instances.
>> This is necessary to ensure that Qdisc is destroyed after all instances of
>> chain/proto that it contains in order to prevent use-after-free error in
>> tc_chain_notify_delete().
>
>
> Please avoid async unless you have to, there are almost always bugs
> when playing with deferred workqueue or any other callbacks.

Indeed, async Qdisc and tp deallocation introduces additional
complexity. What approach would you recommend to make chain_head_change
callback atomic?

>
> Thanks.

Thank you for reviewing my code!